A data network is said to guarantee In-Order Delivery (IOD) if the packets sent by a source host are received in the same order at the destination host.
Most data networks are not required to guarantee in-order delivery. For example, in IP networks, the packets sent by a source host can be received out of order at a destination host. Such networks typically employ a higher layer protocol like Transmission Control Protocol (which uses a re-sequencing buffer at the destination host), to guarantee in-order delivery of packets to the host applications.
However, there are some networks (primarily Layer 2 (L2) networks) that are required to guarantee In-Order Delivery (IOD). Fibre Channel (FC) is an example of such an L2 network. Some FC capable end host devices expect the FC network to deliver packets in order. Out-of-order delivery of packets to such hosts can result in catastrophic failures. With Input/Output (IO) consolidation, newer L2 networks like Data Centre Ethernet (DCE) also need to guarantee IOD. Hence, IOD is a very important feature of new L2 networks.